In U.S. Pat. No. 3,981,481 granted Sept. 21, 1976 assigned to the same assignee as the present application, a bottom operable tank car lading loading and unloading valve is disclosed. Other bottom operable tank car lading valves are disclosed in U.S. Pat. Nos. 3,227,101; 3,591,131; 3,661,355; 3,721,424. These bottom operable tank car lading valves allow the operator to operate the lading valve without going on top of the car. Thus the danger of the operator falling off the car while operating the lading valve is reduced when such a bottom operable lading valve is utilized.
However during loading and unloading a tank car it is necessary to provide a valve to allow air to enter the car during unloading and allow air to leave the car while the car is being loaded. In the past most such air inlet and air outlet valves have been located on top of the car, and operable from the top of the car. Thus the operator must climb on top of a car in order to operate most of the prior art air inlet and outlet valves during loading and unloading the car. Thus to improve operator safety it would be desirable to make the air inlet and outlet valve operable from the bottom of the car.
In U.S. Pat. No. 3,486,528 an air inlet and outlet valve is disclosed which is located on top of the tank. An operating shaft extends from the bottom of the tank up through the tank to the air inlet and outlet valve, and the valve is operable from the bottom of the tank, thus avoiding the need for the operator to go on top of the car to operate the valve.
However when a railway tank car is impacted, the top of the tank moves downwardly relative to the bottom of the tank for up to three inches or more. If the bottom operable valve structure shown in U.S. Pat. No. 3,486,528 were utilized in a railway tank car, when the car is impacted, the top of the tank would move downwardly relative to the bottom of the tank, the valve would open, resulting in the loss of vapors and/or liquid lading.